Experts sift for tiny details at Boston bomb scenes

BOSTON -- Police Commissioner Edward Davis has called it the "most complex crime scene" in the city's history.

Pieces of material that could be key to the investigation may be incredibly small, and those who passed through the area immediately after the bombs went off made the investigation process much more difficult, criminologists said Wednesday.

The investigation is incredibly painstaking, with a need to preserve and document all evidence and scour every inch of the area for clues, some of which may be hardly perceptible to all but the most well-trained eyes, experts said. But it is often the smallest pieces of evidence -- a fragment of a bomb, fibers of a shirt or a piece of a footprint -- that lead investigators to a suspect.

"They cannot be anything but extraordinarily thorough and meticulous," said Rosanna Cavallaro, a professor at Suffolk University Law School in Boston. "There may be things they miss that may connect the dots.

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"Right now," she said, "the richest source of evidence is the material that they're finding."

Teams of investigators continued Wednesday to pore over a span of blocks of Boylston Street, with the perimeter of the crime scene under tight security.

Crews dressed in white suits, with gloves and coverings for their feet, combed the area, taking evidence in what looked to be medal containers. They appeared to then swab the ground underneath.

It's a very precise process, said Isaac Borenstein, a retired judge and adjunct law faculty at Suffolk. Every bit of evidence taken must be documented for where it was found and what it was near, and it must be kept from contamination during the entire process.

"The complicating factor in an urban environment is that people work and live there, and there's a lot of pressure to get this done as soon as you can and get everyone on with their lives," said Jack McDevitt, an associate dean at the Northeastern University School of Criminology and Criminal Justice.

They are likely the best such investigators available in the country, who will have access to the best labs and experts, criminologists said.

A crime scene in a vacuum is an investigator's dream, Cavallaro said, but the Boylston Street site is practically the opposite. Winds whipped through the area also on Tuesday, in addition to the large crowd, further changing the scene.

"One of the fortunate things is it didn't rain," McDevitt said.

Even doctors working on the injured collect evidence when they remove shrapnel or other fragments from their patients. Many emergency responders instinctively kept onlookers and others away from the scene immediately after those injured were helped, Borenstein said.

"It's always a problem," he said of potential contamination during such disasters. "But it's a problem that essentially comes with the turf because the first priority is to get people help."

Even after an arrest in the case, the evidence gathered at the scene would be needed by prosecutors at trial.

Once the crime scene is cleared, a specialized crew is brought in to clean everything and return it to normal. For a crime scene such as the one in Boston, it's a task that likely requires four or five three-person crews working around the clock for two or maybe three days, said Bryan Reifsteck, the director of marketing for Aftermath, a Chicago-area firm that specializes in such projects.

"It's literally starting at one end and combing through to the other end," he said.

The Boylston Street scene is being treated as a biohazard because of a mix of blood and objects like broken glass, Reifsteck said. Any materials must be put in regulated containers and brought to a medical waste facility and decontaminated and property disposed.

Cleanup is made complicated not only by the size of the scene but the materials that need to be cleaned - porous surfaces such as sidewalks and asphalt, said Reifsteck. Thousands of towels will need to be used, and then contained and disposed.